Envy the tiger and the zebra no longer. You have stripes of your own.
Human skin is overlaid with what dermatologists call Blaschko’s Lines, a pattern of stripes covering the body from head to toe. The stripes run up and down your arms and legs and hug your torso. They wrap around the back of your head like a speed skater’s aerodynamic hood and across your face. Or they would, if you could see them.
In the early 1900s, German dermatologist Alfred Blaschko reported that many of his patients’ rashes and moles seemed to follow similar formations, almost as though they were tracing invisible lines. But those lines didn’t follow nerves or blood vessels. They didn’t represent any known body system.
Here is how Blaschko depicted these lines in an early paper:
It turns out these lines are far more extensive than even Blaschko thought.
And today we know what they are: cellular relics of our development from a single cell to a fully formed human. Each one of us started out as a single cell, and then a little glob of cells. As the cells divided, they differentiated. Some became muscles, others bones, still others organs. And some became skin. As those skin cells continued dividing, they expanded and stretched to cover a quickly growing body. One cell line pushed and swirled through another like steamed milk poured into an espresso to make a latte.
Blaschko’s lines are the molecular evidence of those swirls.
Most people will never see their own stripes. As Blaschko noted, there are dozens of skin conditions that follow these lines, but most of them affect patches of skin or a single body part, not the entire body.
And then there are the chimeras. Remember the single cell that turned into a glob? From time to time, two of these starter cells will merge and become a glob together. The glob eventually resolves into a chimera: an animal with two lines of DNA. As the animal’s skin develops, the two groups of cells divide and swirl just like non-chimera skin cells. The difference is that the two groups of chimerical cells are slightly different from each other.
Sometimes, this difference is obvious. More often in humans, though, it’s too subtle to notice with the naked eye, and can only be spotted under UV light.
A version of this story ran in 2015; it has been updated for 2022.